Guanidinium and related compounds have been discovered to cause blockade of gramicidin channels in lipid bilayers. The mechanism of binding and blocking by these molecules will be studied using electrophysiological, spectrosopic, and theoretical techniques. Membrane thickness will be varied to evaluate the geometry of the lipid membrane near the channel entrance. Competition between ions and blocking molecules for the gramicidin channel ion-binding site will be examined to evaluate the binding selectivity of the site. Low-conductance variant channels (minis) formed by gramicidin have blocking characteristics which differ from those of typical gramicidin channels, which will be examined in an effort to determine the cause of decreased conductance in minis. analogues of gramicidin with larger pore diameters will be incorporated into bilayers and the blocking characteristics measured in order to test theories about the blocker's binding site. Acyl guanidinium compounds block sodium channels in nerve and acetyl choline channels in neuromuscular junction with a higher potency than guanidinium. The lipid bilayer - gramicidin system will be used to measure the potency of acyl guanidinium compounds as channel blockers. Because in this system the structure of the lipid and protein are well known, this study will allow a detailed examination of the determinants of blocker potency. The results will be compared to the blocks induced by quanidine-like compounds in the channels from oocytes and other tissues. Such information will contribute to the understanding of excitable membrane function and the design of drugs which can serve as specific channel blockers.